The present invention relates to perfluoropolyalkylether (PFPAE) formulations having enhanced resistance to fluid oxidation and to the corrosion of metals. In particular, the present invention relates to additives to be used with a PFPAE base fluid, as well as procedures for synthesizing these additives.
Perfluoropolyalkylethers (PFPAEs) have long been of interest as lubricant base fluids because of their good thermal and oxidative stability and their wide liquid range. These properties make them particularly well suited for advanced high-efficiency turbine engines for aerospace applications. The practical utility of the commercial fluids for aerospace and military applications has been limited due to corrosion of metal components exposed to these fluids under extreme use conditions.
Deficiencies in lubricant base fluids have generally been mitigated and performance improved by the use of various additives. Conventional additives developed for improvement of a variety of specific properties of petroleum-derived or synthetic hydrocarbon base fluids are generally not suitable for perfluorinated fluids, because of a lack of solubility and efficacy. One method for making the conventional additives soluble in perfluorinated fluids is to synthesize molecules having the same functional groups, but with perfluoroalkyl or perfluoroalkylether moieties. While this generally improves the solubility of the compounds, it typically changes the properties of the critical functional groups and renders the additives ineffective. These difficulties are well known to those familiar with the art. In spite of these difficulties, a few useful additives have been developed for perfluorinated fluids.
An early class of such additives improving the oxidation/corrosion properties of PFPAE oils and greases, comprised compounds described as perfluoroalkylether (PFAE) substituted arylphosphines. While these compounds are still of interest, other more easily synthesized, and sometimes more effective, classes of compounds have been disclosed in more recent patents. Closely related to the phosphines are the perfluoroalkyl- and perfluoroalkylether-substituted aryl phosphates and phosphonates, which have been claimed to be effective as multi-purpose additives.
Fluorinated derivatives of phenoxyphosphazenes were first disclosed as fluids or additives to improve friction and wear properties in U.S. Pat. No. 5,015,405. This same class of compounds was later found to improve resistance to oxidation and corrosion in PFPAE fluids as disclosed in U.S. Pat. No. 5,441,655. Likewise, PFAE substituted benzothiazoles and bis-benzothiazoles were initially suggested as fluids (U.S. Pat. No. 3,666,769) and were later found to be effective additives for the prevention of oxidation and corrosion.
Another class of compounds that has been found to improve resistance to oxidation and corrosion in PFPAE fluids is the PFAE-substituted s-triazines. It was also found that diphenyl ethers substituted only with perfluoroalkyl groups were soluble in PFPAE fluids and provided a measure of protection against oxidation-corrosion stability (U.S. Pat. No. 5,302,760).
All of the foregoing additives, when dissolved in PFPAE fluids, reduce fluid oxidation and reduce the corrosion of certain metal components exposed to the fluids at high temperatures in an oxidative environment. However, all are limited in usefulness by an inability to provide protection from oxidation and corrosion up to about 330.degree. C., or by being particularly difficult and expensive to synthesize, or both. It is desirable to have an additive that minimizes high-temperature oxidation-corrosion reactions in PFPAE fluids at the highest temperature possible, yet is simple and inexpensive to synthesize and have sufficient solubility at sub-ambient temperatures.
Accordingly, it is an object of this invention to provide an easily synthesized, novel oxidation-corrosion additive for enhancing the stability of PFPAEs.
It is another object of the invention to provide a lubricant composition that can be used in the temperature range -40.degree. C. (-40.degree. F.) to 330.degree. C. (625.degree. F.) causing little or no corrosion to ferrous alloys.
It is yet another object of the invention to provide a lubricant composition which is stable to thermo-oxidative degradation when exposed to metals at up to about 330.degree. C. (625.degree. F.).
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.